The present invention pertains to high voltage electric bushings and, more particularly, to high voltage electric bushings of the gas insulated type. The bushing of this invention finds particular application in electrical systems operated at 500 KV and above. Various bushings for supporting high voltage conductors, and for controlling the electric field around such conductors, are known in the art. One design includes a hollow, elongated insulator or weathershed which houses an electric conductor coextensive therewith. When installed, one end of the weathershed is placed adjacent a grounded support surface, and the other end of the weathershed is placed adjacent portions of a high voltage electrical system which are connected to the conductor. Thus, voltage stress is imposed along the length of the weathershed, at its outer surface. It is important that the concentration of voltage stress at any point along the outside surface of the weathershed not exceed known maximum levels, above which failure of the weathershed is known to occur. Unfortunately, the electrical stress is not uniform throughout the length of the weathershed, but is concentrated at regions adjacent the base of the weathershed. Electrically conducting grounded shields, disposed between the conductor and the weathershed, have been proposed as one solution to relieve the high electrical stress adjacent the base of high voltage bushings. This does not, however, offer a satisfactory solution to the problem, since the point of maximum electrical stress experienced by the weathershed is merely displaced to a point where the ground shield is terminated, and the electrical stress concentrations imposed by operating voltages of 500 KV and above are not alleviated.
It has been known that electrical stress on the outside of the weathershed could be relieved if the outside surface of the weathershed could be further separated from the external conductor, as by increasing the internal diameter of the weathershed, or by increasing the wall thickness. Neither alternative offers a practical solution, as weathersheds having large internal diameters are prohibitively costly, as are very thick weathersheds having commercially feasible internal diameters.
It is therefore an object of the present invention to provide a high voltage bushing having improved voltage stress relief means.
Another object of the present invention is to provide an economical high voltage bushing having an enlarged outside diameter adjacent one end thereof to provide electric field stress relief.
It is another object of the present invention to provide a bushing having a weathershed of substantially constant thickness throughout its length.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
Briefly, a high voltage bushing is provided with a hollow, elongated weathershed formed of epoxy, ceramic, porcelain, or the like dielectric material. The weathershed houses a high voltage conductor which extends the entire length thereof. The weathershed, which is of substantially constant thickness throughout its length, receives a collar of substantially greater thickness at one end. The collar is formed of cast epoxy, ceramic, porcelain, or the like high voltage dielectric material, and when installed on the weathershed, imparts increased dielectric strength to the bushing. An interface material, disposed between the collar and the weathershed, provides bonding of the two members, as well as precluding moisture or the like contaminant from entering therebetween. The collar may include a conductive stress relief arrangement, such as a grounded shield, or capacitive foils disposed within its interior.